Artificial guiding star
An artificial guide star or laser guide star (English Laser Guide Star , LGS) is a point of light generated by a laser beam in the upper atmosphere . As an “artificial star”, it enables the air turbulence to be corrected by adaptive optics , even in places in the sky where no correspondingly bright natural guide star (NGS) is available.
Guide stars for adaptive optics
Adaptive optics systems on large astronomical telescopes require the light spot of a natural or artificial guide star in order to measure the turbulence of the layers of the atmosphere above the telescope. The light from this guide star is captured with the telescope and the distortions of its wave front are measured with a wave front sensor, for example a Shack-Hartmann sensor . In this way, the information is obtained in order to correct the wave front of the actual target object via an electronic control system and a deformable mirror. This leads to sharper images of objects outside the atmosphere ( astronomical objects , but also earth satellites). With good correction, the image quality is no longer determined by the seeing , but by the diffraction of the light on the telescope mirror.
Since the distortions caused by the unrest in the air change very quickly, the guide star must be bright in order to deliver enough photons for many wavefront measurements in one second. It must also be close to the target object in the isoplanatic spot , as otherwise the light from the guide star and from the target object would pass through different turbulent areas of the atmosphere on the way to the telescope, so that the distortion measured at the guide star would not be applicable to the target object.
It turns out that for most areas of the sky, particularly far away from the star-rich plain of the Milky Way, no sufficiently bright natural guide star is available. For rare object classes this would be a major limitation of the adaptive optics.
Laser guide stars
This is remedied by laser guide stars, with which a bright laser beam is sent in the direction of the target object with a small telescope lens on the main telescope.
Rayleigh laser guide stars use a laser in visible light or near ultraviolet , the light of which is backscattered by Rayleigh scattering on molecules and aerosols in the lower 10-20 km of the atmosphere. This initially creates a column of light, but by using pulsed lasers and utilizing the transit time of the light, this can be limited to a light spot in a layer of the atmosphere.
With sodium laser guide stars, a pulsed or continuous laser beam with the wavelength of the sodium D-line (589.2 nm) is used instead. The laser beam is scattered back in the atmosphere at a height of approx. 90 km from the sodium atoms in the sodium layer .
Since the laser beam used is also deflected by the layers of the atmosphere below, a normal star is usually observed in addition to the laser guide star. With the help of this star, however, instead of the entire wavefront, only the shift of the observed image as a whole has to be corrected; a relatively weak star is sufficient for this.
history
The first successful project of a laser guide star was carried out for military purposes in the 1980s at the Starfire Optical Range in New Mexico and was initially subject to confidentiality. The first civil experiments on astronomical telescopes began in the 1990s. Since 2000, laser guide stars have also been installed on telescopes of the 8 m class such as the Keck Observatory and the Very Large Telescope .
See also
literature
- Claire E. Max et al .: Observing Techniques for Astronomical Laser Guide Star Adaptive Optics. In: Proc. SPIE. Volume 3353, 1998, pp. 277-281, abstract , PDF .
- Ian S. McLean: Laser Guide Star Systems. In: Ian S. McLean: Electronic imaging in astronomy - detectors and instrumentation. Springer, Berlin 2008, ISBN 978-3-540-76582-0 , p. 63 ff.
- Nancy Ageorges: Laser guide star adaptive optics for astronomy. Kluwer, Dordrecht 2000, ISBN 0-7923-6381-7 .
Web links
- Commissioning of a laser guide star at the VLT
- Adaptive Optics Online Tutorial: Laser Guide Stars
- Laser Guide Star Adaptive Optics @ keck.hawaii.edu, accessed February 15, 2011